MicroevolutionCh. 20

What Is Microevolution?• Evolution a change in a species

genes over time– Time needed various between

populations– Different populations can

experience different changes• Microevolution change in the

genes in one specific population– Easier to monitor– Can lead to speciation

• Monitored through phenotypic variation (changes in appearance) and genotypic variation (changes in DNA sequence)

• All changes must be inheritable for evolution to occur

Ways to Describe Variation• Quantitative variation one trait

expressed in a range; ex. Height, weight, size, etc…– Average of population fits in the

middle of the extremes (bell curve)

• Qualitative variation one trait expressed in only a set number of ways; blood type, flower color, etc…– No intermediates (mixing of traits) – Polymorphism

• Phenotype ≠ Genotype– 50% A type blood doesn’t mean

50% iAiA

Sources of Genetic Variation • New phenotypes do not mean

a change in the populations genome– Phenotypic changes can be

caused by the environment– Artic Hare white color in cold

temperatures1) New alleles mutations or

new DNA from new members

2) Rearrange old alleles 3 methods of variation from meiosis; 10600 combinations of human DNA

Population Genetics• Gene Pool total genome in

a population • Genotype frequencies

percent genes occur in population– Three types (AA, Aa, aa)

• Allele frequencies percent alleles occur in population– Two types (A or a)– AA, Aa, aa 50% A and 50% a

• p= % of A• q= % of a

Hardy-Weinberg Equation• p2 + 2pq + q2 = 1 p + q = 1– p2 AA– 2pq Aa– q2 aa

• What is the frequency of heterozygotes if the dominate phenotypes occur at 19%?– Dominate phenotypes = p2 + 2pq = 0.19– q2 = 1- 0.19 = 0.81 q = 0.9– p + q = 1 p = 1-0.9 = 0.1– 2pq = 2(0.1)(0.9)= 0.18

Hardy-Weinberg Equation• A plant species ability to grow in nickel contaminated

soil is determined by a dominate allelei) If 60% of the plant seeds grow in the soil, what is

the frequency of this resistant allele?ii) Of the plants that grow, what percent are

heterozygous?i) p2+2pq= 0.6 ; q2= 1-0.6 = 0.4; q= 0.63 1-q = p = 1-0.63= 0.37ii) p2 + 2pq= 0.6; p=0.37 p2 = (0.37)2 = 0.14 2pq = 0.6 – 0.14= 0.46

The Null Hypothesis• What must you always have

in a controlled experiment?– A control group

• How can we compare a group of flowers in the wild to a control group? There is no control group in nature

• Null Models predicts outcomes if no factors of effected genetic variation

• Hardy and Weinberg designed the conditions that must ALL occur for genetic equilibrium to occur– Hardy-Weinberg Principle

Hardy-Weinberg Principle• ALL of the following must

occur for a population not to change:

1) No mutations 2) No migration3) Infinite in size4) All genotypes will survive

and reproduce5) All members mate randomly• No population, even a

controlled one, will ever have all of these traits– Example of a perfect control

group in genetics

Causes of Microevolution1) Mutation direct change in the

genome by altering the DNA sequence

• Deleterious mutations that produce harmful effects

• Lethal cause the organism to die before reproducing; often die in early development

• Neutral neither helpful nor harmful • Advantageous positive effect; might

be promoted in future generations• Mutations in somatic cells do not

effect evolution. Why?– Only mutations in gametes get passed

on

Causes of Microevolution2) Gene Flow immigration of new alleles into the population through new members– Must reproduce with old

members to introduce new alleles

– If the new members are very similar to the old population there may be little effect

– Very different new members can cause a huge shift

Causes of Microevolution3) Genetic Drift random events the remove members from the population

– genes do not effect survival– Small populations are greatly

effected; large populations experience little effect

• Population Bottleneck:– Major disaster removes a large

part of the population, those left are the only source of variation

• Founder Effect: – Members that start a new

population in a new area are the only source of variation

Causes of Microevolution4) Natural Selection benefited traits to a certain environment lead to the increased survival rate of that individual

– Increase the appearance of those traits (relative fitness)

• 3 types:1) Directional one extreme of

trait is favored2) Stabilizing intermediate of

extremes of the trait is favored; most common

3) Disruptive both extremes of trait are favored; least common

Causes of Microevolution5) Sexual Selection males/females can be selected for mating based on traits• Intersexual males/females selected

based on useless traits– Color patterns, mating songs, etc…

• Intrasexual males/females use traits to compete for access to each other – Body size, tusks, etc…– Sexual Dimorphism males and females

differ greatly in size• In both cases, if the trait is too extreme

the individual will have limited survival– Too brightly colored= easy to find– Too big of antlers= too hard to move– Balance is best

Balancing Polymorphism • Recessive traits survive

selection through carriers• Even negative mutation are

conserved if the heterozygous form has an advantage

• Sickle Cell Anemia:– HbS HbS normal blood; can

get Malaria– HbA HbA sickle blood; can’t

get Malaria– HbS HbA defective blood,

but resistant to Malaria– Heterzygous is best in malaria

ridden environments

Frequency-Dependent Selection • Advantage of a trait can

change depending on how often it appears

• Water Boatmen:– Insect species with

different shades of brown– Darkest are easiest to

see; least advantages– Dark drop and light

becomes common, common are the focus of predator fish making darker advantages

Path of Adaptations• The cause for adaptations is

not always clear• Current adaptive traits may

have originated from environments very different from current ones

• Traits may have had different uses when first forming

• Bone structures in shoulders use by all birds for flight formed long before birds existed

• Passed on from small dinosaurs and used by birds for flight

Homework• Suggested Homework:– Test Your Knowledge

Ch. 20• Actual Homework:– Discuss the Concepts #4– Design the Experiment – Apply Evolutionary

Thinking